1. Field of the Invention
This invention relates to a method and device for manufacturing a bellows pipe.
2. Description of the Prior Art
In the field of a method and device for manufacturing a bellows pipe, there are known Japanese Patent Publication No. 44-24746 (Bulge machining device), Japanese Patent Laid-Open No. 59-133021 (Method and device for manufacturing corrugated pipes), Japanese Patent Laid-Open No. 63-207421 (Method and device for manufacturing beaded pipe), etc.
Among them, according to Japanese Patent Publication No. 44-24746, a desired bellow portion is shaped in one step by expanding a bead portion using an inner pressure and concurrently causing axial compression using a metallic mold.
According to Japanese Patent Laid-Open No. 59-133021, two core rods with an elastic body interposed therebetween are inserted into a pipe, one core rod is pressed toward the other to deform the elastic body, a portion of the pipe is withdrawn to form an expanded angle portion, and the expanded portion is shaped to conform to a bend mold, resulting in a corrugated pipe.
According to Japanese Patent Laid-Open No. 63-207421, a pressure medium is arranged inside a work to give an inner pressure thereto, metallic molds defining a bead forming recess thereinside are provided outside the work relatively movably in the axial direction of the work, the work is put in a plastic deformable or expandable state, and the metallic molds are made close to each other, resulting in a beaded pipe.
Specifically, in the system of Japanese Patent Publication No. 44-24746, the metallic mold must be prepared so as to meet the length of the pipe and the number of beads of the bellows portion; thus, the deformable length of the pipe is limited and the metallic mold is expensive.
In the two systems of Japanese Patent Laid-Open No. 59-133021 and Japanese Patent Laid-Open No. 63-207421, the degree of freedom relating to the number of beads in the bellows portion is comparatively large; but, the pipe length, bead count and bead position are limited by reason of the structure of the core rods in the former case or of the structure of the metallic mold mounting portion in the latter case.
Further, in the latter two systems, the outer diameter and pitch of the bellows portion are limited by reason of bellows forming grooves formed in the metallic mold, thereby diminishing the degree of freedom relating to the shape of the bellows portion. In addition, since both ends of a pipe portion to be expanded are fixed by the metallic mold, the pipe cannot shift sufficiently in the axial direction during expanding, a bead top portion becomes thin, and the elastic body is severely worn because sliding be required between the pipe and the elastic body.
Further, in the latter two systems, to prevent the elastic body from protruding from a chuck portion of the metallic mold, the force of holding down the metallic mold must be set large; consequently, an excessive force is imposed on the elastic body to promote the wearing thereof, and the elastic body is compressed from the side of the molded bellows portion during continuous molding; thus, the elastic body eats into a gap of the bellows portion, thereby being damaged.
Further, in the system of Japanese Patent Laid-Open No. 63-207421, since the expanded portion together with the elastic body is compressed in the axial direction in the step of changing the expanded portion to the bellows portion, the elastic body tends to be worn or torn off, making the shape of the bellows nonuniform.